A Hemolysin is secreted by Staphlococcus aureus as a water-soluble monomer, which upon binding to the surface of a susceptible cell, such as that provided by a human red blood cell, the monomer oligomerizes to a heptamer and forms a transmembrane pore approximately 15 A in diameter. The membrane-embedded toxin initially causes leakage of cellular contents with molecular weights below 300 daltons and finally gives rise to cell lysis. We have recently solved the structure of the detergnet-solubilized heptamer using multiple isomorphous replacement and density modification methods at 2.4 A resolution. All 293 amino acids have been built into the electron density map and we are ready to begin crystallographic refinement using the computer program X-PLOR. We plan to refine the structure of the heptamer at 2.4 A resolution and to solve the heptamer structure based on data collected to 1.6 A resolution at -160 C. To carry out the refinements we will use the program X-PLOR.